Construction member

ABSTRACT

A construction member comprising a core of plastic foam or expanded plastic material, a covering of plastics resin reinforced with glass fibres bonded to the faces of the core, reinforcing rods being disposed at the connection of the covering with the core such that upon hardening of the resin the reinforcing rods are tightly bonded to the core as well as the covering. The construction member may be formed as a panel, paving slab, flooring member, telephone pole, or silo depending on the desired use.

United States Patent 1191 Allard Feb. 12, 1974 CONSTRUCTION MEMBER2,850,890 9/1958 Rubenstein 161 /161 3,000,144 9/1961 Kitson l6l/l23[761 lnventm- Franc Avenue 3,139,243 6/1965 Lux 1 1 161/161 Leclere,130ml, France 3,331,173 7/1967 Elsner l6l/DlG. 4 3,556,9l8 l/197lLemelson l6l/l59 [22] July 1971 3,574,104 4/1971 Medler 1151/60 [21]Appl. No.: 163,297

Primary ExaminerWilliam J. Van Balen [30] Foreign Applicafinn PriorityData Attorney, Agent, or FirmWaters, Roditi, Schwartz &

Nissen July 22, 1970 France 70.27047 Jan. 15, l97l France 71.01277ABSTRACT 52 us. c1 161/160, 52/309, 161/60, A Construction membercomprising a Core of Plastic 1 2 16l/142, 161/16l foam or expandedplastic material, a covering of plas- 5 Int C] 3 3 2 1332b 5/18, 1332b 5tics resin reinforced with glass fibres bonded to the [58] Field ofSeal-chm" 161/60, 160, 142, 161, faces of the core, reinforcing rodsbeing disposed at 52/309 the connection of the covering with the coresuch that upon hardening of the resin the reinforcing rods are [56]References Cited tightly bonded to the core as well as the covering. The

UNITED STATES PATENTS construction member may be formed as a panel, pav-3 043 730 7/1962 Ad 161,161 ing slab, flooring member, telephone pole,or silo de- 1e 3,640,787 2/1972 Heller l6l/l6l pending on the deslreduse.

3/l958 Robinson 161/123 18 Claims, 18 Drawing Figures sa lz PMENTEDFEBI219m SHEU 1 [if Q mammal 2:914

' SHEET M 0F Q l mm? CONSTRUCTION MEMBER The present invention relatesto construction members.

Construction panels of various types having a plastic material base usedin the construction of dwellings and the manufacture of containers areknown.

The present invention concerns, more particularly, panels having on bothsides of a core of expanded or foam plastic material coverings orcoatings of a plastic material, preferably polyester reinforced withglass fibres.

These panels have advantageous structural characteristics which areascribed principally to the tightness of the bond between the coveringsthe core and which is obtained in the course of manufacture during thehardening of the resin used for the coverings or coatings.

However, such panels particularly when they are of a small thicknesshave insufficient bending strength for certain applications.

It has been attempted to increase the structural strength of thesepanels by providing internal stiffening members but the manufacture ofsuch panels is complicated because it is necessary to divide the coreinto sections that are disposed on both sides of the stiffeners whichproduces a panel which is not entirely satisfactory.

The present invention is directed to the manufacture of suchconstruction members wherein at the connection between the body of themember and the covering or skin there is provided a metal reinforcingmember incorporated therein by the curing of the resin whichsimultaneously assures the tightness of the bond between the coveringand the reinforcing member, the covering and the body as well as thereinforcing member and the body.

The reinforcing members advantageously comprise metal reinforcing rodsused in reinforced concrete construction in which the bars are notsubjected to prior tension.

Although reinforcing metal members have been used for quite some time inreinforced concrete construction in placing them in the forms into whichthe aggregate is poured, and eventually the reinforcing members undertension, in a manner so as to be fixed therein upon the hardening of theconcrete, it was not seen how to use such reinforcing members in panelmembers of the above described type in which the core of body is made ofpreformed plates of expanded plastic material in which such reinforcingmembers could not be arranged, no less be fixed to the core withoutunacceptable complications.

The present invention overcomes this difficulty by incorporating thereinforcing members in the covering or coating, or skin, and takesadvantage of the fact that during the manufacture of the panel, thecovering is formed from a resin which is not yet solidified and includesflexible glass fibres disposed in several layers.

After the polymerization of the resin, the reinforcing member isperfectly fixed to the elements constituting the covering or coatingwhich explains the improved structural characteristics of the panel thusformed.

In a construction member according to the invention, the body or core ofexpanded material or foam effectively resists the shearing stresses andfor certain shapes further prevents the composite covering from bucklingor busting.

The covering or skin is itself a composite or layered material having avery small thickness and collects and transmits the shearing stress.

The reinforcement provides the member with qualities resulting from itsexcellent modulus of elasticity. The reinforcement is used in a smallvolume and its distance from the neutral axis renders it particularlyeffective.

The combination of these three components enable the forming of memberswhich are heat insulating, owing to the presence of the foam and theabsence of thermal bridges, shock resistant because of the properties ofthe steel reinforcing members and the layered structure, and fluidtightbecause of the layered structure.

Sound insulation may be increased by the juxtaposition of an appropriatecomponent on one of the faces.

The incorporation of the reinforcing metal members enables the thicknessof the covering to be relatively small, all other things remainingconstant.

The member is corrosion-proof. Members can be formed which are planar,curved or linear acting in bending (with or without shear stress) and/orin compression and/or in torsion.

The invention concerns plates and sheets including parallel reinforcingmembers, posts and beams with peripheral reinforcing membersadvantageously placed at an angle and possibly connected by frames andbraces or yokes.

Such members according to the invention have a resistance to strainswhich is not provided by interior structures, as usual, but by theexterior portions, the interior acting as the body.

The members according to the invention enable the construction ofmembers acting principally in bending such as for roofing, facings,gables, ceilings suitable for required loads, as well as members actingprincipally in compression and flexure such as walls, partitions,dividers, supports and finally members acting principally in tension andcompression such as silos.

The design of these members may be carried out by conventional methods.

In an embodiment according to the present invention, the body is ofextruded plastic material having a surface of a greater density than itsinterior such as can be obtained directly from an extrusion die. A flatconstruction member is therefore provided in which one of the facesformed has a surface of extruded plastic material with a greater densitythus taking advantage of the particular properties thereof.

In the description which follows, made by way of example, reference willbe made to the accompanying drawings, in which:

FIG. 1 shows a view in section of a panel according to the inventionduring its manufacture in an apparatus;

FIG. 2 shows a plan view of the upper face of the core with one of thegrooves being filled;

FIG. 3 shows a view in section of a panel according to the invention;

FIG. 4 shows a view in section similar to FIG. 3 but for an alternativeembodiment;

FIG. 5 shows a view in perspective with a portion in section of afurther embodiment;

FIG. 6 shows a view of a slab according to the invention;

FIG. 7 shows a view in cross-section on a larger scale of a portion ofthe slab;

FIG. 8 shows a perspective view of a flooring member of a transportcontainer;

FIG. 9 shows a perspective view of a body forming a part of a memberaccording to the invention;

FIG. 10 shows a perspective view of reinforcing members for use in thismember;

FIG. 11 shows a perspective view of a panel according to the invention;

FIG. 12 shows a perspective view of the same member but wiewed from theother face;

FIG. 13 shows a perspective view of a panel according to the inventionfor another embodiment;

FIG. 14 shows a partial view in cross-section of a silo according to theinvention;

FIG. 15 shows a view in section taken along line 15--15 in FIG. 14 on amuch larger scale;

FIG. 16 shows a view in cross-section of a vertical support or poleaccording to the invention;

FIG. 17 shows a view in section of a transport container panel accordingto the invention; and

FIG. 18 is an horizontal section of a container panel for anotherembodiment.

Reference will be made to FIG. 1. On the wall 10 of a mould or table,after the application or a stripping agent, a layer 11 of an agent suchas a gel coat or finishing agent is applied. One or more layers 12 of aglass fabric or web impregnated with resin, advantageously withnon-solidified polyester resin is set on the layer 1 l. A plate 13 ofexpanded plastic material, advantageously polyvinyl chloride is placedon the layers 12.

The plate 13 has on its face 14, opposite its face 15 in contact withthe layers 12, grooves 16 (FIG. 2) parallel to one another obtained bymachining or in the course of manufacturing the plate.

In the grooves 16 are placed, without prestress, concrete reinforcingmembers 17 preferably with ribs or small projections as usual andwithout previous tension, in contradistinction to certain reinforcedconcrete techniques. On the face 14 are placed layers 19 of glass fabricor a web impregnated with non-solidified resin. The entire combinationis then covered by a mould cover 20 having a face 21 in contact with thelayers 19; the face 21 may have received a coating 22 of a finishingagent or gel coat.

According to a modification, the reinforcing members are disposed in thegrooves after they are filled with resin, the resin impregnated cloth ormat being placed on the plate thus provided with reinforcing members.

In another modification, the second facing is effected in a second mouldwhich covers the first mould after the completion thereof.

The moulding operation may include pressurization and increase oftemperature.

After removal from the mould, a panel is obtained having structuralcharacteristics which are clearly greater than those of known panels ofthe same type not having any reinforcing members. This result comes fromthe tightness of the bond between the reinforcing members and thecoating(s) or covering( s) of the panel assured by the resin which is incontinuous contact with the reinforcing members as well as with theglass fabric which in turn is intimately connected to the plate 13.

In another embodiment, the manufacture of the panel is effected withoutusing a mould.

In another embodiment shown in FIG. 3 the panel comprises a core 23 ofexpanded polyvinyl chloride with a coating or covering 24 of polyesterreinforced with glass fibres on one face and a coating or covering 26 ofpolyester reinforced with glass fibres on the other face 25, reinforcingmembers 28 being incorporated in the panel on the interior face 27 ofthe covering.

The face of the panel thus reinforced is the one intended to be subjectto tension when the panel bends.

The presence of the reinforcing members also increases the strength ofthe panel in compression.

In the embodiment shown in FIG. 4 the panel 30 comprises a first seriesof parallel reinforcing members 31 incorporated in the covering 32 andthe core and a second series of reinforcing members 33 incorporated inthe covering 34 of the opposite face and the core.

In the embodiment shown in FIG. 5, the panel comprises a core 41 ofexpanded polyvinyl chloride, a covering or facing 42 having parallelreinforcing members 43 incorporated according to the above describedmethod. The other face of the core 41 is covered by a plate 44 offibrocement, the facing 45 opposite the facing 42 being of polyesterreinforced with glass fibres.

Such a panel is particularly useful as a paving slab.

Reference will now be made to FIGS. 6 and 7. The embodiment shown inthese figures comprises a panel body laterally flanked by two beams orribs 51 and 52. The body 50 comprises a core 53 of expanded polyvinylchloride which has a face 54 with parallel grooves or slots 55.Transverse reinforcing members 56 are housed in the grooves. The otherface 57 of the core 53 has transverse grooves staggered relative to thegrooves and in which are arranged reinforcing members 58 having the samelength as the reinforcing members 56.

A beam 51 comprises a first block 59 of expanded polyvinyl chloridewhich projects from the core 53 and a second block 60 also of expandedpolyvinyl chloride. The first block has outer longitudinal channels 61and 62 at its outer corners and the second block 60 has channels 63 and64 in its outer corners. In the channels 61-64 are arranged reinforcingmembers 65-68, of the type used for reinforced concrete.

A coating or covering or skin 69 of polyester reinforced with glassfibres forms the outer surface of the member and also contributes to theconnection between the core 53 and the reinforcing members 56 and 58 aswell as between the blocks 59 and 60 and the reinforcing members 65-68.The polyester resin and the glass fibres covering the reinforcingmembers also contribute to the connection of the reinforcing memberswith the adjacent block of expanded polyvinyl chloride.

Such a construction member is particularly useful as a slab.

Reference will now be made to FIG. 8. In this embodiment the panel 71per se comprises a core 72 of expanded polyvinyl chloride withlongitudinal reinforcing members 73 incorporated in the said core andthe covering, (removed in the Figure), on one face and longitudinalreinforcing members 74 staggered relative to the reinforcing members 73arranged on the other face.

Cross beams 75 have cores 76 of expanded polyvinyl chloride (assumedtransparent in the figure), and these beams are connected to the panel70 by a covering or coating of resin reinforced with glass fibres. Inthe outer cores of the beam 75 are disposed reinforcing members 77 and78 incorporated in the core 76 and the covering 79 as previouslyindicated.

The reinforcing members of the type used for reinforced concrete mayalso be provided in the inner corners of the beams 75.

Such a construction member is particularly useful as a containerflooring member.

The construction member according to the invention is useful as a rulein all the cases where panels are presently employed having a core oflaminate wood with faces covered by a covering of plastic materialreinforced with glass fibres, for example for the construction oftransport containers which are more and more wide-spread in the field ofground or sea transport. For the same strength, the panels according tothe invention are approximately half as heavy as such laminated panelswithout the production cost being increased.

The member according to the invention may also be used in theconstruction of panels,'doors, gables, roofs, for forming slabs, roofingmaterials, buildings and housing units, for the construction ofhatchways for refrigerated boats, vehicle flooring, etc.

Its superior structural features thereby enable the use of panels havinga lesser thickness.

In a modification, the core is of an expanded plastic material otherthan polyvinyl chloride, for example, polyurethane or polystyrene or aphenolic foam.

The expanded plastic core may be formed in situ.

Reference will now be made to FIGS. 9-12. The body 101 is an article ofthermoplastics material such as obtained by extrusion with a relativelythin outer layer 102 superficially hard and strong of non-expandedmaterial whereas the interior is of expanded or foam material.

Parallel grooves 104 with semi-circular crosssections are formed on oneof the faces of the body 101 from extrusion or by subsequent operationwith milling cutters.

Reinforcing members 105 of the type used in reinforced concrete,preferably having small projections 106, are disposed in the grooves104. At the outset the reinforcing members 105 are coated in a mixtureof hardenable resin and fibres, preferably glass fibres, or the groovesare filled with such a mixture before the reinforcing members arearranged therein.

On the body 101 thus formed is applied a composition comprising ahardenable resin with glass fibres. The fibres may constitute one ormore layers. It is left to harden. The hardened resin is not onlyintimately bonded to the fibres but also with the reinforcing members105 and the body 101. A construction member is thus obtained havingexceptionally increased structural strength which comes from thequalities of the body 101 having a hard layer 102 as well as thereinforcing members and the outer composite layer 107 resulting from thehardening of the resin with glass fibres.

In order to manufacture panels of larger dimensions, a certain number ofbodies 101 and 102 etc. are disposed side by side (FIGS. 11 and 12).After being provided with reinforcing members 105, resin is put betweenthe adjacent edges and a composite covering is then provided which notonly ensures the reciprocal incorporation of the reinforcing members inthe bodies 101 and the coating 107 but also contributes to the joiningof the juxtaposed bodies to one another in order to obtain a panel ofthe desired width.

The manufacturing apparatus may comprise a table on which is applied agel coat, then the resin, then glass fabric or webs, finally thepreviously extruded article provided with reinforcing members whichcomes into contact with the resin.

The combination is then put under pressure, for example, by theapplication of a plastic sheet under vacuum conditions.

In certain cases the polymerization is accelerated by heating.

The panel has excellent structural characteristics in addition to thequalities of the extruded article itself. Its manufacture is simple andits production cost is low.

The members according to the invention may be used for the constructionof walls for transport containers, railway cars, or all other vehicles,also for constructing vehicles and constructing flooring, roofs, doorsand walls, etc..

Reference will now made to FIG. 13. In this embodiment the constructionmember comprises a panel 111 formed as has been indicated by thejuxtaposition of a plurality of members 101 with reinforcing members105, the composite covering of resin and glass fibres being furtherapplied for the bonding of the panel with a body 112 of lighter materialand having greater thermal and sound insulation characteristics, forexample expanded polyvinyl chloride.

Reference will now be made to FIGS. 14 and 15 which show a silo formedfrom members each comprising a body 201 of expanded plastic materialhaving an annular cylindrical shape with a covering on its outer andinner faces 202 and 203 respectively composed of a resin reinforced withfibres such as glass fibres. The member has for reinforcements an outerreinforcing member 204 arranged as a helix at the junction between thecovering 202 and the body 201 and bonded to the body and to the coveringby the resin itself which forms part of the covering.

This construction may also comprise an inner reinforcement comprising areinforcing member 205 formed as a helix and arranged at the connectionbetween the body 201 and the covering 203 and bonded thereto by theresin itself which forms the covering.

Reference will now be made to FIG. 16 relative to a pole or verticalpost for use, for example, for supporting electric or telephone lines.The pole 201 comprises a body 210 of expanded plastic material shaped asa prism, for example, with a square cross-section, and having a covering211 of a resin with glass fibres on its lateral faces. At the corners ofthe post in the connecting zone between the body 210 and the covering211 are arranged vertical reinforcing members 212-2ll5 which are bondedto the body 210 as well as to the covering 211 by the resin of which thecovering is formed.

Reference will now be made to FIG. 17 which schematically illustrates insection a panel intended to constitute the wall of a transportcontainer. According to the invention the reinforcing members 220provided at the connection between the body of expanded plastic materialof the panel and one or both of the two coverings of a resin with glassfibres are distributed in a way as to correspond to the forces to whichthe panel is to be subjected. Accordingly, in a central zone 221 thereinthe reinforcing members are very close together and in two marginalzones 222 and 223 the reinforcing members are not quite so closetogether as in the central zone, spaces being also provided between thezones 222 and 223 and the zone 221 having the reinforcing membersrelatively spaced apart. A panel is thus provided which is ofexceptional lightness in spite of its very high structural strength.

In all cases the reinforcing members or rods may have non-circularcross-sections.

In the embodiment shown in FIG. 18, the reinforcing members have anelongated cross-section, rectangular or elliptical as shown at 230 andthe reinforcing members have a helicoidal rib 231. Each member, in thisembodiment is in contact relationship with one and the other externalcoating or skin 233 and 234 in its opposite apex zones 235 and 236. Thebody or core 237 of the element is of expanded polyvinyl chloride. Theresin of the composite material made of glass fibres and resin, which ispart of the coatings ensures the rigidity of said material both in thecore 237 and members 230.

Such a wall presents, with small thickness, a particularly goodresistance.

It is advantageously used for the manufacture of containers.

In the embodiments contemplated for the construction of parts for whichfire proofness is required, the invention contemplates phenolic resinsfor the constitution of the coatings or skin.

Panels according to the invention can be made in various thickness froml2-l5mm up to 15 cm or more.

The expanded plastic material constituting the core of the panel has adensity of 0.03 to 0.15 the compression, shear or tension resistancestrength being between l-2 kg/cm and about 10 kg/cm The thickness of acovering or skin is about 1 to 2 The weight of a panel having athickness between mm and 2-3 cm is less than 10 kg/m If necessary,panels of very great surface areas, up to 50 m long and 2.5 m width canbe manufactured.

What I claim is:

l. A construction element comprising a core of expanded resin materialhaving a hardened foam structure with an outer peripheral surface, saidcore being substantially homogeneous and having little compressivestrength, said core defining a a pre-formed recess extendinglongitudinally thereof at said peripheral surface, a covering of resinmaterial reinforced with fibres bonded on said outer peripheral surfaceof said core and extending into said recess thereof, said coveringhaving an outer surface opposite said outer peripheral surface of thecore, and an elongated metal reinforcing bar disposed in said recess tosubstantially fill the same and be at the junction between the coveringand said outer peripheral surface of the core in substantial proximityto the outer peripheral surface of the element, said bar beingnon-pretensioned and extending substantially coextensive in length withthe core, said recess being relatively shallow in relation to the barsuch that the covering of resin material forms a relatively uniform thinskin extending over the entire peripheral surface of the core inclusiveof said bar, said bar being bonded to the covering and thereby to saidcore by the resin of said covering.

2. A construction element as claimed in claim 1 wherein said pre-formedrecess is a groove in the peripheral surface of the core.

3. A construction element as claimed in claim 1 wherein said fibers areglass fibers.

4. A construction element as claimed in claim 2 wherein said fibers areglass fibers, a layer of said fibers impregnated with resin beingarranged at the bottom of the groove.

5. A construction element as claimed in claim 1 wherein a plurality ofsaid recesses are provided in said core, said recesses beingsubstantially parallel, a plurality of said reinforcing bars beingprovided, one in each recess.

6. A construction element as claimed in claim 1 wherein said core hasopposite outer surfaces and is provided with one of said coverings oneach opposite peripheral surface with reinforcing bars at both junctionsof the core with said coverings.

7. A construction element as claimed in claim 1 wherein saidconstruction element is annular in shape and said reinforcing rod ishelically wound in said element.

8. A construction element as claimed in claim 5 comprising a second ofsaid coverings on said core on the inner peripheral surface thereof, anda second helically wound reinforcing bar in a recess provided at theinner peripheral surface of said core.

9. A silo comprising at least one construction element according toclaim 8.

10. A construction element as claimed in claim 1 of slab shape wherein aplurality of said reinforcing rods are mounted in respective of saidrecesses at said outer peripheral surface of the core, and furthercomprising a fibrocement plate bonded to said core on the face thereofopposite that containing the reinforcing rods.

11. A transport container formed of panels each comprising aconstruction element as claimed in claim 1.

12. A slab comprising a panel including a construction element asclaimed in claim 5, said panel having opposite edges, beams mountedalong said edges of the panel and integral therewith, said beamsincluding bodies of expanded resin material having an outer surface andprovided with corners, second reinforcing bars at said corners of thebeam, and a covering of resin material reinforced with fibers bonded tosaid bodies on the outer surface thereof and bonding said secondreinforcing bars to said bodies and to said covering.

13. A flooring member comprising a panel including a constructionelement as claimed in claim 1, and cross beams on one face of said panelhaving cores of expanded resin material with second reinforcing barstherein.

14. A construction element as claimed in claim 1 wherein saidreinforcing bars are of the type used for reinforced concrete.

15. A panel comprising a plurality of construction elements as claimedin claim 1 in juxtaposed relation, the covering of resin and reinforcingfibers being common to the justaposed elements and assembling the same.

16. A pole for carrying electric power or telephone lines comprising aconstruction element as claimed in claim 1 of elongated shape.

wherein the core of resin material has a density of about 0.03 to 0.15and a compression strength less than 10 Kg/cm.

2. A construction element as claimed in claim 1 wherein said pre-formedrecess is a groove in the peripheral surface of the core.
 3. Aconstruction element as claimed in claim 1 wherein said fibers are glassfibers.
 4. A construction element as claimed in claim 2 wherein saidfibers are glass fibers, a layer of said fibers impregnated with resinbeing arranged at the bottom of the groove.
 5. A construction element asclaimed in claim 1 wherein a plurality of said recesses are provided insaid core, said recesses being substantially parallel, a plurality ofsaid reinforcing bars being provided, one in each recess.
 6. Aconstruction element as claimed in claim 1 wherein said core hasopposite outer surfaces and is provided with one of said coverings oneach opposite peripheral surface with reinforcing bars at both junctionsof the core with said coverings.
 7. A construction element as claimed inclaim 1 wherein said construction element is annular in shape and saidreinforcing rod is helically wound in said element.
 8. A constructionelement as claimed in claim 5 comprising a second of said coverings onsaid core on the inner peripheral surface thereof, and a secondhelically wound reinforcing bar in a recess provided at the innerperipheral surface of said core.
 9. A silo comprising at least oneconstruction element according to claim
 8. 10. A construction element asclaimed in claim 1 of slab shape wherein a plurality of said reinforcingrods are mounted in respective of said recesses at said outer peripheralsurface of the core, and further comprising a fibrocement plate bondedto said core on the face thereof opposite that containing thereinforcing rods.
 11. A transport container formed of panels eachcomprising a construction element as claimed in claim
 1. 12. A slabcomprising a panel including a construction element as claimed in claim5, said panel having opposite edges, beams mounted along said edges ofthe panel and integral therewith, said beams including bodies ofexpanded resin material having an outer surface and provided withcorners, second reinforcing bars at said corners of the beam, and acovering of resin material reinforced with fibers bonded to said bodieson the outer surface thereof and bonding said second reinforcing bars tosaid bodies and to said covering.
 13. A flooring member comprising apanel including a construction element as claimed in claim 1, and crossbeams on one face of said panel having cores of expanded resin materialwith second reinforcing bars therein.
 14. A construction element asclaimed in claim 1 wherein said reinforcing bars are of the type usedfor reinforced concrete.
 15. A panel comprising a plurality ofconstruction elements as claimed in claim 1 in juxtaposed relation, thecovering of resin and reinforcing fibers being common to the justaposedelements and assembling the same.
 16. A pole for carrying electric poweror telephone lines comprising a construction element as claimed in claim1 of elongated shape.
 17. A pole as claimed in claim 16 wherein saidconstruction element has a cross-section of prism shape with one of saidreinforcing bars being mounted in a recess provided at each of the edgesof said prism.
 18. A construction element as claimed in claim 1 whereinthe core of resin material has a density of about 0.03 to 0.15 and acompression strength less than 10 Kg/cm2.